The present invention relates to offshore structures used in the petroleum industry and particularly to floating structures, for example, some semi-submersible drilling vessels or floating platforms that are used for production platforms. An example of a floating production platform would be a tension leg platform used to develop fields that are located in water depths beyond the economic feasibility of fixed leg platforms.
Floating platforms, used for production or drilling and production of a deepwater field, may have relatively deep drafts, for example in excess of 45 feet. The deep draft of these vessels limits the areas of the world in which the vessels can be fabricated, since the launching of the vessel requires a deepwater facility. One solution to the requirement of a deepwater building facility has been to build the hull and the deck components separately and then assemble them in a deepwater location. In this procedure, the hull is fabricated and towed to a deepwater location. The deck is separately fabricated and loaded on a suitable barge and towed to the deepwater location. The hull is then ballasted and submerged until only the uppermost portion of the supporting columns extend above the surface. The deck is then maneuvered into place over the support columns and the hull deballasted to raise it so that the columns can engage the deck and lift the deck from the barge. The deck barge may also be ballasted down as a part of this process to aid in the load transfer. The barge is then removed from between the support columns of the hull and the attachment of the hull to the deck completed. This type of procedure is described in U.S. Pat. No. 3,797,438 where the hull and deck are built separately and then assembled in a deepwater location.
While the method has been used to assemble the hull and deck of semi-submersibles and other offshore structures, it is extremely difficult and involves risk to both the equipment and the personnel carrying out the work. It can be readily appreciated that any wave, current or wind forces that cause either the hull or the barge transporting the deck to move can lead to damage to the other structure if they collide as a result of the movement. For example, any rise or fall of the deck of even a few inches can cause severe damage to the supporting columns on the hull as it is being deballasted to raise the columns into engagement with the deck. In addition, the problem of accurately aligning the support structure with the support columns on the hull involves considerable time and effort. Various structures with procedures for use have been developed to cushion or absorb some of the shock of the mating of the hull and deck in these operations. For example, U.S. Pat. No. 4,848,967 describes a shock absorbing system that is used reduce the shock as an integrated deck structure is lowered onto a fixed offshore platform substructure. As shown in the '967 patent, the deck is transported by a barge which is then ballasted to lower the deck structure onto the substructure.
At the present time, there is considerable interest in developing deepwater petroleum reserves in the Gulf of Mexico. This development requires the use of tension leg platforms which normally have a draft of greater than 45 feet. Most of the waterways in the Gulf area are limited to a 40-foot depth and thus the structure cannot be assembled in a building yard and towed to the desired location as a completely assembled unit. In order to build a structure in the Gulf area it is necessary to build the deck and the hull separately and tow each unit to a deepwater location. The depth must be sufficient to allow submerging of hull structure and the floating of the deck structure onto the support columns of the hull as described above. This requires a considerable period of good weather to complete the assembly to the stage necessary for the structure to float freely as a unit. Considerable time can be lost while waiting for a suitable weather pattern to develop that will allow assembly of the structure in the deepwater location.